Glaserite is the double salt of potassium and sodium sulfate. Chemically, the mineral has the formula K3Na(SO4)2 and is a common intermediate compound in the production of potassium sulfate. Potassium sulfate is a frequently-used specialty potassium fertilizer for certain agricultural crops, especially those crops that are sensitive to chloride.
U.S. Pat. No. 1,936,070 discloses several processes for producing glaserite. In one process, solid sodium sulfate is added to a strongly concentrated or saturated solution of potassium chloride in water. A second process is to add solid potassium chloride to a strongly concentrated or saturated solution of sodium sulfate in water. In a third process, glaserite can also be produced by dissolving both potassium chloride and sodium sulfate in water, then evaporating the water to cause glaserite to crystallize, with the evaporation continuing until the byproduct sodium chloride becomes saturated in the solution.
Glaserite produced by any of these processes can be of sufficient purity for further processing into potassium sulfate product without further purification. All three processes described above share a common chemical reaction:3KCl+2Na2SO4→3NaCl+K3Na(SO4)2 All three of the above processes typically require potassium chloride and sodium sulfate in the form of finished or nearly finished products as raw materials. This can make the aforementioned processes relatively expensive.
Another route for the production of glaserite is solar evaporation of certain naturally occurring brines that contain sodium, potassium and sulfate. Examples of such brines are those in Searles Lake, Calif. and the brine in Utah's Great Salt Lake. The solar evaporation of naturally occurring brine, such as the examples given, has the advantage of having a very low cost raw material for the constituents in glaserite. However, glaserite produced by these methods is typically mixed with several undesirable salts, for example, sodium chloride. These undesirable salts typically must first be separated to produce a purified glaserite of a quality that is suitable for further processing into potassium sulfate. U.S. Pat. No. 3,675,773 describes flotation separation of glaserite from sodium chloride and other salts.
Yet another process to produce glaserite is the solvent extraction process to produce boric acid from complex brine, for example the brine in Searles Lake, as described in U.S. Pat. Nos. 2,969,275, 3,111,383 and 3,479,294. In this process a byproduct mixture that contains both glaserite and anhydrous sodium sulfate is produced in addition to the principal boric acid product. This mixture of sulfate salts contains from 25% to 75% glaserite with the balance being anhydrous sodium sulfate.
A traditional process for recovering potassium sulfate from glaserite is described in U.S. Pat. No. 1,936,070. In the process of this patent, glaserite is leached with a saturated solution of potassium chloride in water. This patent suggests that the leaching should typically occur at normal room temperature, between 15-40° C., although temperatures outside this range were said to produce product of satisfactory quality. Following the teachings in this patent a digestion-leaching process can be operated to replace substantially all of the sodium in the glaserite with potassium.
The leaching process described in this patent has two process inefficiencies. First, the leach solution leaves saturated in potassium sulfate and glaserite. This means that part of the sulfate in the glaserite feed to the leach process is dissolved into the leach brine and is no longer available to make potassium sulfate product. If the leach occurs at 35° C. and the molar ratio of sodium/potassium in the glaserite is the theoretical 0.333, the dissolution of sulfate will result in about 5.3% of the yield of potassium sulfate being lost to the end liquor.
The other inefficiency is that the potassium concentration in the spent leach brine is quite high because the solution leaves saturated in potassium sulfate and glaserite. This means that the process efficiency for potassium is relatively low. If the leach occurs at 35° C. and the molar ratio of sodium/potassium in the glaserite is the theoretical 0.333, the residual potassium in the spent leach brine will still be about 71% of the potassium supplied in the potassium chloride leach brine.
As the sodium content in the glaserite increases due to the solid solution of sodium sulfate, the inefficiencies described above also increase, because more water must be used to carry away the additional sodium. When the molar ratio of sodium/potassium in the glaserite is 0.50, the loss of sulfate will be about 8% and the residual potassium in the spent leach brine will increase to about 81% of the potassium supplied in the potassium chloride leach brine.
U.S. Pat. No. 1,936,070 describes the use of the mother liquor that results from leaching glaserite to potassium sulfate to produce additional glaserite. Sodium sulfate can be added to this mother liquor to precipitate the contained potassium as additional glaserite. Doing so can recover much of the residual potassium in the mother liquor resulting in a substantially higher overall potassium yield than otherwise possible. However, the added sodium sulfate must be relatively pure and be especially low in sodium chloride. The sodium sulfate is therefore relatively costly.
The second step conversion of glaserite to potassium sulfate as described in U.S. Pat. No. 1,936,070 cannot achieve high yields of potassium and sulfate unless it is coupled with a first step where glaserite is recovered from the mother liquor that results from leaching glaserite to potassium sulfate. If glaserite is recovered from other sources, such as solar ponds or other process, it will be expected to be the principal raw material for the production of potassium sulfate. As such it would be undesirable to produce glaserite from the potassium sulfate mother liquor as well. The process in U.S. Pat. No. 1,936,070 would also require the supply of a large amount of potassium chloride to digest the glaserite to potassium sulfate and it may be very undesirable to have to supply this costly raw material.